Recently an oxyfuel combustor has been reviewed as one of techniques for treating carbon dioxide (CO2) which is said to be one of factors for global warming, and attention has been attracted to, for example, a coal-fired boiler for oxyfuel combustion of pulverized coal. It has been conceived in such coal-fired boiler that oxygen in lieu of air is used as an oxidizing agent to produce combustion exhaust gas mainly composed of carbon dioxide (CO2) and such exhaust gas with high CO2 concentration is compressed and cooled into liquefied carbon dioxide; such liquefied carbon dioxide is transported by a vessel, a vehicle or other carrying means to a destination for storage thereof in the ground or alternatively the liquefied carbon dioxide increased in pressure is transported through a pipeline to a destination for storage thereof in the ground.
Such exhaust gas from the coal-fired boiler for oxyfuel combustion contains, in addition to carbon dioxide (CO2), impurities derived from coal feedstock such as nitrogen oxides (NOx), sulfur oxides (SOx), hydrargyrum (Hg), hydrogen chloride (HCl) and dust.
Among the above-mentioned impurities, sulfur oxides (SOx) may be contacted with and dissolved in water into sulfuric acid (H2SO4) and hydrogen chloride (HCl) may be dissolved in water into hydrochloric acid, so that such water-soluble sulfur oxide and hydrogen chloride as well as dust may be disunited through contact with water.
Among the nitrogen oxides (NOx) as the above-mentioned impurities, nitrogen dioxide (NO2) may be contacted with and dissolved in water into nitric acid (HNO3) to become disunited. However, the exhaust gas from the coal-fired boiler has less oxygen (O2) so that nitrogen (N2) exists substantially in the form of nitrogen monoxide (NO) which is water-insoluble and thus is unremovable by water spraying or the like.
The above-mentioned sulfuric acid, hydrochloric acid and nitric acid are known to corrode instruments in the exhaust gas treatment device; hydrargyrum, which is trace metal, is known to hurt low-temperatured aluminum members constituting a heat exchanger. Thus, it is preferable to remove these impurities in the exhaust gas at early stages. There is also a problem that admixture of the impurities into the exhaust gas lowers a purity degree of the carbon dioxide, which makes troublesome the liquefaction of the carbon dioxide through compression and cooling and thus requires larger-sized equipment therefor. Thus, in a coal-fired boiler for oxyfuel combustion or other system where exhaust gas mainly composed of carbon dioxide is produced and the carbon dioxide is disposed, it is extremely important to remove impurities in the exhaust gas.
Thus, it has been conducted, for example, in the coal-fired boiler for oxyfuel combustion that a spray-column-type, packed-column-type or other wet desulfurizer used in a conventional air-fired boiler or the like is provided to remove sulfur oxides which are especially problematic in corrosion. Moreover, nitrogen and nitrogen oxides derived from coal feedstock are produced in the exhaust gas from the coal-fired boiler for oxyfuel combustion or the like, so that it has been conducted that a catalyst-type or other denitrator is arranged upstream of the desulfurizer to remove the nitrogen and nitrogen oxides.
It is known in the wet desulfurizer as mentioned in the above that sulfur oxides and hydrogen chloride as well as dust are removed and that nitrogen oxides are partly removed and hydrargyrum, which is inherently low in content, is slightly removed. It has been also conceived that if hydrargyrum in the exhaust gas is still high in concentration even after the above-mentioned exhaust gas treatment is conducted, a hydrargyrum-removing column is arranged to remove the hydrargyrum by adsorbent or the like.
As mentioned in the above, exhaust gas mainly composed of carbon dioxide (CO2) from a coal-fired boiler for oxyfuel combustion usually undergoes compression by a plurality of compressors, cooling by aftercoolers respectively downstream of the respective compressors and liquefaction into liquefied carbon dioxide by cooling using a heat exchanger. In this case, there is a problem that the compressors may be corroded by sulfuric acid (H2SO4) specifically resulting from sulfur oxides (SOx) included in the exhaust gas. It is a very important task to prevent the compressors, which are extremely costly, form being corroded.
An exhaust gas treatment system comprises, for example, a duct for guidance of exhaust gas from a boiler which burns fuel with combustion gas, i.e., a mixture of oxygen-rich gas with circulated exhaust gas, the duct being provided with a dust collector and a wet desulfurizer, an exhaust gas recirculation duct for guidance of part of the exhaust gas downstream of the dust collector to the boiler, CO2 separation means for compression of the exhaust gas downstream of the desulfurizer to separate carbon dioxide, water separated in the compression of the exhaust gas by the CO2 separation means being supplied to absorbing liquid used in circulation in the desulfurizer (see Patent Literature 1).
In addition, there are exhaust gas treatment systems disclosed in Patent Literatures 2-5.